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Preparation to move XkbTracker out from components/exo/wayland.
This CL now wraps the dependency to xkbcommon from
wayland_keyboard_delegate.cc

BUG=1123705
TEST=Built locally.

Change-Id: If732a97aff7ccf729f60ea17fc1b193f3244b275
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416153Reviewed-by: Jun Mukai <mukai@chromium.org>
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808727}